Synthesis and Ultrahigh Pressure Compression of High-Entropy Boride (Hf(0.2)Mo(0.2)Nb(0.2)Ta(0.2)Zr(0.2))B(2) to 220 GPa

The high-entropy boride (Hf(0.2)Mo(0.2)Nb(0.2)Ta(0.2)Zr(0.2))B(2) material was synthesized under high-pressures and high-temperatures in a large-volume Paris-Edinburgh (PE) press from a ball-milled powder mix of HfO(2), MoO(3), Nb(2)O(5,) Ta(2)O(5), ZrO(2), carbon black, and boron carbide. The transformation process was monitored in situ by energy-dispersive x-ray diffraction with conversion starting at 1100 °C and completed by 2000 °C with the formation of a single hexagonal AlB(2)-type phase. The synthesized sample was recovered, powdered, and mixed with platinum pressure marker and studied under high pressure by angle-dispersive x-ray diffraction in a diamond anvil cell. The hexagonal AlB(2)-type phase of (Hf(0.2)Mo(0.2)Nb(0.2)Ta(0.2)Zr(0.2))B(2) was found to be stable up to the highest pressure of 220 GPa reached in this study (volume compression V/V(0) = 0.70). The third order Birch-Murnaghan equation of state fit to the high-pressure data up to 220 GPa results in an ambient pressure unit cell volume [Formula: see text] [Formula: see text] = 407 ± 6 GPa, pressure derivative of bulk-modulus [Formula: see text] = 2.73 ± 0.045 GPa. Our study indicates that this high-entropy boride (Hf(0.2)Mo(0.2)Nb(0.2)Ta(0.2)Zr(0.2))B(2) material is stable to ultrahigh pressures and temperatures and exhibit high bulk modulus similar to other incompressible transition metal borides like ReB(2) and Os(2)B(3).